For the measurement of filling levels measuring systems are often used which operate by measuring the delay required by an electromagnetic wave sent out from a filling level sensor attached in the vessel cover to reach the filling matter surface and return. When the height of the vessel is known, the desired filling level can be computed. Such sensors, also known as filling level radar in the pertinent art, usually rely on the fact that electromagnetic waves propagate at constant speeds within a homogeneous non-conducting medium and are at least partially reflected at the boundary surface of differing media. Each boundary layer of two media with different dielectric constants causes a radar echo of the incident wave. The greater the difference in the two dielectric constants, the greater the change in wave resistance of the wave propagation and the stronger the echo that will be observed.
To determine the desired wave delay, different radar principles are known. Two frequently used techniques are on the one hand the impulse delay technique (impulse radar) and on the other hand the frequency modulated continuous wave technique (FMCW radar). The impulse radar relies on pulse form amplitude modulation of the wave to be radiated and determines the direct time duration between transmitting and receiving the impulse. The FMCW radar, on the other hand, determines the delay in an indirect manner by transmitting a frequency modulated signal and determining the difference between a transmitted and a received momentary frequency.
Apart from the various radar principles, depending on the application, various frequency areas of electromagnetic waves are used. There are, for example, impulse radar devices with carrier frequencies in the area of between 5 and 30 GHz and also those which work in the base band as so-called mono pulse radars without a carrier frequency.
A series of methods and devices is also known which guide electromagnetic waves to the filling matter surface and back. In principle, a distinction can be made between waves radiated into space and waves guided by a conductor. A filling level measuring device in which microwaves are guided via a coaxial conductor into an antenna for radiating an electromagnetic wave, is known, for example, from EP 0 834 722 A2. Herein, the antenna is configured to be bipartite and comprises a first antenna portion in the form of a solid cylinder of a dielectric material surrounded by a metal sleeve. Adjacent to this first antenna portion is a second antenna portion which radiates the wave signal towards the filling matter. The filling level measuring device itself is attached within a vessel nozzle by means of a flange attachment, wherein a separate sealing element is inserted to provide a seal between the vessel flange and the housing flange.
The filling level sensors working according to the principle of delay measurement of guided electromagnetic waves, also referred to as time domain reflectometry (TDR) filling level sensors, have a different structure with respect to guide-through and signal guidance. In these filling level sensors an electromagnetic wave is guided via a conductor, such as a probe in the form of a metallic cable or rod, to the point of reflection and from there back to the sensor.
These sensors have considerably less damping of the reflected echo signals than those which freely radiate high frequency waves, because the power flows only in the very limited area in the vicinity along the conductive wave guide. Interfering echoes from within the vessel, such as caused by reflections of the wave on structures within the vessel (stirring apparatus, tubes), and which, with freely radiating sensors, make the identification of the one echo from the filling material surface difficult, are largely avoided with sensors having guided waves. This means that the filling level measurement using guided electromagnetic waves is largely independent of the vessel construction and also of the product quality of the filling matter or other operating conditions (e.g. dust, angle of bulk material) and thus leads to reliable measuring results.
Any known conductors of high frequency waves may be used as the wave conductor for guiding the wave, wherein the wave at least partially penetrates the medium which surrounds the metallic conductor or is surrounded by it. Due to its simple mechanical construction and its suitability for all kinds of filling matter, i.e. bulk materials and liquids, the single-wire conductor or the single conductor probe is often used in the filling level measuring field. When it is configured as a rod or cable probe, it is particularly insensitive to filling matter build-up and attachment. DE 44 04 745 C2 describes an exemplary filling level sensor with such a probe.
The conduction path between the electronics and the probe with such filling level sensors almost always comprises the above-mentioned guide-through assembly and an additional coaxial cable which establishes the connection to the sensor's electronics on which the electronic circuit for generating the transmission signal and the evaluation of the reflected signal resides. The coaxial cable can be eliminated in some special cases where the printed circuit board has a direct electrical and mechanical connection to the guide-through assembly. The guide-through assembly has the function of guiding the measuring signal from the sensor attached on the outside of the filling matter vessel to the probe extending on the inside of the vessel. It must also provide mechanical support to the probe. For this purpose, it usually has a metallic process connection which can be fixedly attached in the vessel, e.g. in a cover opening of the same, and which receives an interior conductor to guide the wave. In order to avoid any short circuits, there is an insulating element between the carrier element and the conductor element. The interior conductor connects the coaxial cable usually leading to the electronics on the one hand and the probe protruding into the vessel on the other hand.
Conventional guide-through assemblies for single-conductor probes usually have a coaxial structure, i.e. the interior conductor is coaxially surrounded by the insulating element and the process connection. While this basic structure can be technically realised in various ways, usually certain requirements must be met, such as sealing the vessel atmosphere, pressure resistance, resistance to high tensile forces on the probe, resistance to high temperatures and resistance against an aggressive vessel atmosphere. In particular, to meet the sealing requirements, usually elastomeric sealing elements must be provided in the guide-through assembly in order to seal the interior of the vessel against the interior of the sensor housing.
The structure of a capacitive filling level sensor is very similar to the above-described structure of a TDR filling level sensor. The measuring principle of such capacitive filling level sensors is based on the fact that the filling matter and the vessel together with the measuring probe form an electrical capacitor. In this measuring technique, the filling level is detected by measuring the capacitor's capacitance, which varies as the filling level varies, and which allows the filling height to be determined.
Such capacitive filling level sensors are known, for example, from DE 027 44 864 A1 or DE 030 50 189 A1, from which the structure of capacitive filling level sensors, which is similar to TDR filling level sensors, can be seen. These sensors also have an internal conductor which is insulated with respect to a process connection. To meet the sealing requirements, also in connection with these capacitive filling level sensors, sealing elements must be provided in the guide-through assembly in order to seal the interior of the vessel against the interior of the sensor. Such sealing elements are usually of resilient, partially crystalline or thermoplastic materials, such as polytetrafluoroethylene (PTFE). However, such materials usually have a drawback in that they begin to get brittle or to flow when exposed to high pressures and/or temperatures, which is why such sealing elements are not suitable for filling level sensors exposed to high pressures and/or temperatures.